Forward collision warning system with road-side target filtering

ABSTRACT

A method of filtering/rejecting targets detected by a forward collision warning system of a motor vehicle when entering a curved road segment. The width of a collision threat zone (CTZ) is reduced in one or more stages depending upon continuously-measured values of steering input angle (SIA), driver brake application (DBA), and a yaw rate. The measured values are used to find: a degree of near-past DBA variation during a first look-back period, a degree of near-past SIA change rate during a second look-back period, and a degree of far-past yaw rate change during a third look-back period longer than the first and the second look-back periods. A series of first, second and third width reductions of decreasing severity are applied to the CTZ based upon comparisons of the tracked variations and change rates with respective thresholds. The thresholds are tunable to achieve desired levels or false-target rejection.

TECHNICAL FIELD

The invention relates to Forward Collision Warning systems for motorvehicles, and to a method for reducing the frequency of false warningsbased on stationary road-side objects detected when the vehicle isdriving on or entering a curved road section.

BACKGROUND

Forward Collision Warning (FCW) generally refers to the use of one ormore forward-looking sensors mounted on a host vehicle to detectobstacles in the host vehicle's path. If a potential collision danger toa sensed in-path target is determined to exist, the system can trigger awarning to help the driver avoid the potential collision. Alternativelyor in addition, the FCW system may trigger an automatic brakingintervention, and/or activate occupant safety systems before a collisionactually occurs if it appears unavoidable.

FCW may be implemented along with adaptive cruise control (ACC) and/orcollision mitigation by braking (CMbB) systems, all of which may utilizea common radar sensor. FCW may typically provide warning for moving andmoveable vehicles, where moveable is defined as a vehicle that haspreviously been tracked by the radar as moving, but has come to a stop.

A camera-based computer vision system may be added to detect lanemarkings to thereby enable lane departure warning. The radar and thecamera may be combined to detect and then classify targets as vehicles.Radar is the primary detection sensor, and computer vision is thevehicle classifier. Computer vision based vehicle classification is usedto expand the operational scope of FCW to include stationary vehicles.Vehicle classification by camera also serves to reduce the likelihoodthat objects typically not in the roadway, e.g., trees, poles andoverhead signs, trigger a false warning.

In systems without a computer vision camera, it would be advantageous todevelop an FCW system capable of operating with radar only yet capableof accurately discrimination between stationary and moving targets.However, without a vision sensor for vehicle classification, non-vehicleobjects such as trees, poles and overhead signs have the potential to beincluded as valid, in-path targets to which FCW may potentially respondby issuing a warning. If the object is not truly in the vehicle path,the resulting warning could be interpreted by the driver as a falsewarning.

One measure of system reliability is the number of false warnings for agiven number of miles driven, or the number of false warnings for agiven test route. As the number of false warnings increases, the systemreliability decreases. If the system reliability is too low, somedrivers may become habituated to ignore all FCW warnings or may turn offthe FCW system altogether and lose the benefits of warnings for truepotential collisions.

Stationary objects may be classified into three basic categories basedon where the object is in relation to the roadway: on-road, overhead,and side-of-path or roadside objects. Roadside objects includesguardrails, roadside signs, trees, reflectors, concrete dividers,manhole covers, storm drain covers and raised lane edge markers (such asBott dots).

Roadside objects may, of course, be present on both straight and curvedroads, however false FCW warnings triggered by roadside objects are moreprevalent in curved road sections. This is because on a straight sectionof road it is relatively easy to estimate the predicted path of the hostvehicle, while when travelling on a curve (or about to enter a curve)the path prediction is more difficult.

An object is considered to be an out-of-path object only when thelateral clearance between the host vehicle and the target object, afterroad curvature is taken into consideration, is larger than half of thehost vehicle width plus half of the target width. However, most radarscurrently considered to be appropriate for use on motor vehicles are notcapable of accurately determining target width. Instead, the radar datamay be used to estimate the target width, or all targets may be assumedto be of a standard width. An object with a smaller width, but laterallyclose to the host vehicle, could be treated as an in-path object andtrigger an FCW warning.

Another factor that may cause out-of-path objects to be falselyidentified as in-path is path prediction error. The lateral clearancebetween the host vehicle and the target object is calculated based onthe predicted host vehicle path. A host vehicle path predictionalgorithm is generally less accurate during steering transitions fromstraight to curved road or vice versa. These path errors and subsequentlateral offset errors can cause out-of-path objects on the side of theroadway to be considered as in the host vehicle's path.

SUMMARY

In a disclosed embodiment, a method of forward collision warning for amotor vehicle comprises operating a sensor system to detect a targetahead of the vehicle and identifying a Collision Threat Zone (CTZ) alonga predicted path of the vehicle. Vehicle parameters are continuouslymeasured, including a steering input angle (SIA) change rate and a yawrate of the vehicle. These measured values are used to continuouslytrack a near-past maximum absolute value of the SIA change rate during afirst look-back period and a far-past variation of the yaw rate during asecond look-back period longer than the first look-back period. A firstwidth-reduction is applied to at least a portion of the CTZ in the eventthat: a) the near-past maximum absolute value of SIA change rate exceedsa first SIA change rate threshold, and b) the far-past variation of yawrate exceeds a first yaw rate threshold value. Threat warnings aresuppressed for any target that is outside of the CTZ with the firstwidth-reduction applied. The first width reduction may comprise reducingthe width to zero, so that a complete block or all threat warnings is ineffect.

In another disclosed embodiment, the method further comprisescontinuously measuring a driver brake application (DBA), continuouslytracking a near-past change in the driver brake application during athird look-back period shorter than the second look-back period; andsuppressing the threat warning if: a) the near-past change of the driverbrake application exceeds a first DBA threshold value; and b) at leastone of the near-past maximum absolute value of SIA change rate and thefar-past variation of yaw rate exceed the first SIA change ratethreshold and the first yaw rate threshold value respectively

In another disclosed embodiment, a method of forward collision warningfor a motor vehicle comprises operating a sensor system to detect atarget ahead of the vehicle, identifying a Collision Threat Zone along apredicted path of the vehicle, and continuously measuring a steeringinput angle (SIA) change rate, a yaw rate, and a driver brakeapplication. Vehicle parameters are continuously measured, including anear-past maximum absolute value of the SIA change rate during a firstlook-back period, a far-past variation of the yaw rate during a secondlook-back period longer than the first look-back period and a near-pastvariation of the driver brake application during a third look-backperiod shorter than the second look-back period. Threat warnings aresuppressed if: a) the near-past variation of the driver brakeapplication exceeds a DBA threshold, and b) the near-past maximumabsolute value of SIA change rate exceeds a first SIA change ratethreshold OR the far-past variation of yaw rate exceeds a medium yawrate threshold.

In another disclosed embodiment, a method comprises operating a forwardcollision warning sensor of a motor vehicle to detect a target ahead ofthe vehicle, identifying a collision threat zone (CTZ) ahead of thevehicle in which the target is considered a collision threat, the CTZhaving a base width, and continuously measuring a steering input angle(SIA) change rate, a driver brake application (DBA), and a yaw rate. Themeasured values are used to find: a degree of near-past DBA during afirst look-back period, a degree of near-past SIA change rate during asecond look-back period, and a degree of far-past yaw rate during athird look-back period longer than the first and the second look-backperiods. A first width reduction is applied to the CTZ if either: 1) thenear past SIA change rate exceeds a peak SIA change rate threshold, or2) the near-past DBA indicates braking; and either a) the near past SIAchange rate exceeds a second SIA change rate threshold lower than thepeak SIA change rate threshold, or b) the far-past yaw rate exceeds afirst yaw rate threshold. Threat warnings are suppressed if the targetis outside of the CTZ with the first width reduction applied.

In a still further disclosed embodiment, the method further comprises,in the event that the conditions to apply the first width reductionabove are not met, applying a second width reduction less restrictivethan the first width reduction to the CTZ when: a) the near-past maximumabsolute value of SIA change rate exceeds the second SIA change ratethreshold, and b) the far-past variation of yaw rate exceeds the firstyaw rate threshold value; and suppressing the threat warning if thetarget is outside of the CTZ with the second width-reduction applied.

In a still further disclosed embodiment, the method further comprises,in the event that the conditions to apply the first and second widthreduction above are not met, applying a third width reduction lessrestrictive than the second width reduction to the CTZ when: a) thenear-past maximum absolute value of SIA change rate exceeds the secondSIA change rate threshold; and b) the far-past variation of yaw rateexceeds a second yaw rate threshold lower than the first yaw ratethreshold.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention described herein are recited withparticularity in the appended claims. However, other features willbecome more apparent, and the embodiments may be best understood byreferring to the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 is a schematic depiction of a collision threat zone (CTZ) of ahost vehicle travelling along a straight road segment;

FIG. 2 illustrates the geometry of a generic curved road section;

FIG. 3 is a schematic depiction of a CTZ of a host vehicle travellingalong a curved road segment;

FIG. 4 is a flow-chart of a method for reducing false FCW warnings; and

FIG. 5 is a subroutine used in the method of FIG. 4.

DETAILED DESCRIPTION

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention that may be embodied in variousand alternative forms. The figures are not necessarily to scale; somefeatures may be exaggerated or minimized to show details of particularcomponents. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as arepresentative basis for teaching one skilled in the art to variouslyemploy the present invention.

FIG. 1 depicts a collision threat zone (CTZ) for a host vehicletravelling along a straight road segment (host vehicle yaw rate equal tozero). The CTZ is in the predicted path of the moving vehicle and isidentified to minimize the number of side-of-path objects that will beconsidered as valid targets. If a target object, moving or stationary,or a significant portion of an obstacle, falls within the CTZ, it isconsidered a valid target. Any target detected that falls outside theCTZ is not considered a valid target, and no FCW warning is issued.

The CTZ is centered on the predicted host vehicle path and projectsforward from the front of the vehicle. The host vehicle's projected pathoriginates from the center of the host vehicle and projects forward in astraight line when the host vehicle is travelling straight ahead. TheCTZ is ideally set to the exact width of the host vehicle. In the mostgeneral case, the predicted path is based on the host vehicle's currentyaw rate and longitudinal velocity and accounts for any turning actionby the host vehicle.

The lateral distance between the centerline of the host vehicle'sprojected path and the target is indicated in FIG. 1 as D_(OFFSET). On astraight road, D_(OFFSET) is simply the host-to-target lateral offset.For a target at the range of L meters and azimuth angle of θ radians,D _(OFFSET) =L sin θ  (1)

To check whether or not a target falls into the CTZ, simply subtracthalf the target width from the lateral offset. The target width forradar-only FCW may be an assumed width if the radar does not providetarget width information, or if the accuracy of the available targetwidth information is less than desired.

FIG. 2 illustrates the geometry of a generic curved road section, arc ABbeing the predicted path of the host vehicle as it follows the road,with radius R. A is the front center point of host vehicle. C is thecenter point of the target vehicle. B is a point on the predicted pathhaving the same range from point A as C.

Road Curvature Angle α is the angle at the host vehicle center A betweenthe tangent of the arc of the predicted path of radius R and line AB.Target azimuth angle θ is the sum of Road Curvature Angle α and angle β.To subtract Road Curvature Angle α from target azimuth angle θ isequivalent to converting D_(OFFSET) for a curved road to D_(OFFSET) fora straight road.

From geometry,

$\begin{matrix}{\alpha \approx {\sin^{- 1}\left( \frac{L}{2R} \right)}} & (2)\end{matrix}$

$\begin{matrix}{D_{offset} \approx {L\;{\sin\left( {\theta - \alpha} \right)}} \approx {L\;{\sin\left( {\theta - {\arcsin\left( \frac{L}{2R} \right)}} \right)}}} & (3)\end{matrix}$

Target offset D_(OFFSET) is shown in FIG. 2 as CB.

The road curvature angle a may be calculated from the measured hostvehicle yaw rate thus:

$\begin{matrix}{\alpha = {\frac{L}{2} \times \frac{\omega}{\upsilon}}} & (4)\end{matrix}$

Where:

L—Target range in meters

ω—Host vehicle yaw rate in radians/s

ν—Host vehicle speed in meters/s

Ideally, this stationary target CTZ discriminates between roadsideobjects (not a collision threat) and stationary objects that are in thetravel lane. However, due to a number of factors (such as noiseassociated with real-world radar sensor data and errors in theprediction of the path of the host vehicle), a significant number ofroadside objects may still be included in the CTZ area and thereforefalsely treated as valid targets.

Several factors may result in loss of accuracy of path prediction,including any inherent measurement error of yaw rate and target azimuthangle. Another potentially significant error inducing factor is thedriver's unintentional movement of the steering wheel (or other steeringinput control, for a vehicle so equipped) during lane keeping.Variations in the steering input angle (SIA) signal can be explained asthe small, random steering corrections. It has been found thatdriver-induced SIA variation alone can cause D_(OFFSET) error at leastin the range of [−0.4. 0.4] meters.

As discussed above, one of the primary causes of false warnings on roadside objects is error associated with host vehicle path prediction basedon yaw rate. When the host vehicle is entering a curve or travellingalong a curve having a decreasing radius, it becomes particularlydifficult to mitigate false warnings caused by road side objects.

False warnings due to road side objects may be reduced or eliminated intwo ways: a) Using an improved path-prediction method to determinewhether objects detected are or are not in the host vehicle's path; andb) Reducing the CTZ width for which targets will be considered valid.The CTZ width reduction may be applied in stages depending oncombinations of parameters (as discussed below) and may include reducingthe CTZ width to zero, the equivalent of completely blocking the FCWwarning. The objective is to maintain as much true warning capability incurves as possible while providing excellent rejection of false warningsfor these scenarios.

Reduction of CTZ width may be achieved by applying a width reductionfilter to all or a portion of the CTZ. An example of such awidth-reduced CTZ is shown in FIG. 3. The width of the CTZ is reducedmore drastically as distance from the host vehicle increases, taperingto a minimum width at the maximum range of the CTZ. Any stationaryvehicle or other objects within the boundary of the reduced width CTZ isconsidered valid by the algorithm and therefore may trigger an FCWwarning.

As discussed earlier, using yaw rate alone to predict the host vehiclepath may lead to high levels of error at the entry to a curve or whenthe radius of the curve is decreasing. Under these conditions, the yawrate may lag changes in SIA (driver movement of the steering wheel, inmost cases), so that SIA may provide an earlier and better estimate ofthe path required to negotiate the curve. Using SIA input to generatethe path prediction has been found to effectively filter out objectsthat will be out-of-path, but still maintain true warnings for objectson curves that are in the host vehicle's path on a curve.

The SIA-based path prediction is calculated by first estimating the hostvehicle yaw rate by measuring host vehicle SIA and host vehicle speed,and entering a look-up table with those values. The look-up table valuesmay be determined empirically by analyzing real-world test data. Theestimated yaw rate from the look-up table is then used to calculate aSIA-based D_(OFFSET), or path prediction, in the same fashion as theD_(OFFSET) calculated from the actual yaw rate signal.

The SIA-based path prediction also includes a minimum SIA threshold,below which the predicted yaw rate is zero, and consequently thepredicted path is straight ahead. This threshold may be necessary toaccount for situations where the host vehicle's steering systemcenter-find algorithm predicts a non-zero SIA when the vehicle isactually traveling straight.

Empirical evidence has shown there to be some variation in theSIA-to-yaw rate ratio from one vehicle to another. The ratio isdependent on a number of vehicle parameters, and can be expected to varywithin the same vehicle design. However, the accuracy of this predictionis not necessarily critical. If the predicted yaw rate and D_(OFFSET)are too large, a true target in a curve could be predicted to be out ofpath and no warning provided. Compared to a countermeasure designwithout a SIA-based path prediction, but still with the goal to minimizefalse warnings at curves, the warning would probably still be missed dueto the driver steering input. If the error is such that the predictedyaw rate and D_(OFFSET) are too small, then a true warning for a targetin the host's path on a curve would still be provided. However, thelikelihood of false warnings for road side objects would increase. Inthis case, if the number of false warnings is acceptable, including theSIA-based path prediction provides improved warning performance incurves, even if it doesn't occur 100% of the time. In other words,providing some true warning performance in curves, even if there is somevariation in performance between vehicles, may still be a preferredapproach to providing no true warnings in these situations.

Studies have indicated that a significant portion of warnings associatedwith curves occur when the yaw rate is one degree/second or greaterand/or SIA is 10 degrees or greater. Such studies have also indicatedthat both yaw rate and SIA are relatively high for false curve entrywarnings on “curvy roads”, where the host vehicle is travelling from onecurve into another. Also, there is generally some SIA input associatedwith many of the false warnings occurring at or just prior to curveentry, where the vehicle was on a relatively straight road prior to thewarning. In many of these cases the yaw Rate is still very low.

Empirical analysis of curve entry events has led to the followingobservations:

Recent history of the yaw rate could be useful for determining that thedriver is in “curvy” road conditions, even if the current yaw rate isvery low;

Recent SIA changes often occurred prior to the warning and provide anearly indication that the host vehicle may be entering a curve;

Recent brake application by the driver of the host vehicle (driver brakeapplication, or DBA), especially associated with a SIA change, is astrong indication that the host vehicle is entering a curve; and

SIA changes and DBA are most useful in the very recent history (lessthan 1 second prior to the warning). The yaw rate, however, is mostuseful looking farther back (up to ten seconds) to determine whether“curvy” road conditions are being experienced.

A method will now be described, with reference to FIG. 4, which makesuse of the above discoveries. Beginning at block 100, the values of SIA,DBA, and Yaw Rate (all of the host vehicle) are continually measured byappropriate sensors.

At block 110, a signal tracking algorithm maintains a signal history foreach of the three measured parameters by storing maximum and minimumsignal values at fixed time intervals (data points), and then rollingthe stored values in a first-in-first-out (FIFO) manner such that theoldest values are thrown away as new values are added. The fixed timeinterval at which the data points are stored is determined by dividingthe time over which the signal history is being tracked (hereinaftercalled the look-back period) by the number of points being tracked. Asan example, ten sets of maximum and minimum values are tracked for the“Far” look-back period, and five sets of maximum and minimum values aretracked for the “Near” look-back period. The fixed time interval betweendata points for the “Far” look-back period is therefore:

$\begin{matrix}{\frac{{Far}\mspace{14mu}{History}\mspace{14mu}{Time}}{10};} & (5)\end{matrix}$

and the “Near” time interval is:

$\begin{matrix}{\frac{{Near}\mspace{14mu}{History}\mspace{14mu}{Time}}{5}.} & (6)\end{matrix}$

For example, if the “Far” look-back period time is ten seconds, then thenew data

$\frac{10}{10} = 1$points are stored every second. If the “Near” look-back period time is0.6 seconds, then the

$\frac{0.60}{5} = 0.120$new “Near” data points are stored every second.

As new data values are continuously added, the signal tracking algorithmoutputs the new maximum and minimum of all the tracked points over theappropriate time interval. For the example numbers above, therefore, new“Far-Max” and “Far-Min” values are provided each second as the maximumand minimum of the ten historical maximum and ten minimum stored datapoints, respectively. Similarly, new “Near-Max” and “Near-Min” valuesare provided every 0.120 second as the maximum and minimum of the fivehistorical maximum and minimum stored data points, respectively.

For the Yaw Rate signal, the final calculation of the maximum signalchange over the look-back period is made by subtracting the minimumvalue during the look-back period from the maximum value during thelook-back period as shown below:Yaw Rate Change “Far”=Yaw Rate Max “Far”−Yaw Rate Min “Far”  (7)

For the SIA Change Rate (SIA_CR) signal, consider the absolute value ofthe “Near” look-back period signals maximum, minimum, and currentSIA_CR, and selecting the largest value. Including the current SIA_CRensures the maximum SIA_CR is always being considered, even if it is thelatest/most current measurement. The SIA_CR “Near” measure is thereforecalculated as:

$\begin{matrix}{{{SIA\_ CR}{``{Near}"}} = {{Max}\left\lbrack {\begin{matrix}{{SIA\_ CR}\mspace{14mu}{Max}\mspace{14mu}{``{Near}"}} \\{{SIA\_ CR}\mspace{14mu}{Min}\mspace{14mu}{``{Near}"}} \\{{SIA\_ CR}\mspace{14mu}{Current}}\end{matrix}} \right\rbrack}} & (8)\end{matrix}$

For Driver Brake Application (DBA), brake application may be assumed tobe binary, such that DBA=0 when no brake applied, and DBA=1 when anybraking is applied. For this assumption, simply check whether a signalchange greater than 0.5 occurred in the “Near” look-back period. Similarto the calculation above of SIA_CR, the “Near” calculation includes thecurrent/most recent DBA measurement. The DBA “Near” look-back periodsignal calculation is:

$\begin{matrix}{{{DBA}\;{``{Near}"}} = {\begin{matrix}1 & {{if}\begin{bmatrix}{{{{DBA\_ Max}\mspace{14mu}{``{Near}"}} - {{DBA\_ Min}\mspace{14mu}{``{Near}"}}} > 0.5} \\{and} \\{{{DBA}\mspace{14mu}{Current}} > 0.5}\end{bmatrix}} \\0 & {Otherwise}\end{matrix}}} & (9)\end{matrix}$

Regarding DBA, it is also possible to monitor/track the degree ofbraking being applied by the driver, rather than the more simple binaryassumption described above.

Beginning at block 120, the algorithm utilizes the signal trackinginformation described above to determine whether to reduce the CTZwidth, and how much width reduction is appropriate. Depending on therecent level of driver steering input (as indicated by the look-backperiod signal, SIA_CR “Near”), whether the driver has recently appliedthe brake (DBA “Near”), and the vehicle's Yaw Rate Change “Far”, thefilter will either:

a) block all warnings (CTZ width reduced to zero),

b) apply a first width reduction to the CTZ (less restrictive than thewarning block),

c) apply a second width reduction (less restrictive than the first widthreduction) to the CTZ, or

d) leave the CTZ width at the full original width.

As the driver input (as measured by the signal histories over thelook-back periods) indicate greater SIA input and driver control, thewidth of the CTZ is reduced so that FCW system warnings only occur inresponse to targets that are very centered in the path of the vehicle.With sufficiently high driver steering inputs, no warnings at all areallowed, so essentially the width of the CTZ is reduced to zero. Thedetails for each of possible CTZ width adjustment actions are providedbelow.

At block 120 the following relationships are evaluated:SIA_CR “Near”≧Peak_CR  (10)

OR

$\begin{matrix}{{{{DBA}\mspace{14mu}{``{Near}"}} > {0\mspace{14mu}{{AND}\begin{bmatrix}{{{SIA\_ CR}\mspace{14mu}{``{Near}"}} \geq {Min\_ CR}} \\{OR} \\{{{YawRate}\mspace{14mu}{``{Far}"}} \geq {Med\_ YawRt}}\end{bmatrix}}}},} & (11)\end{matrix}$

-   -   where the parameters “Peak_CR”, “Min_CR”, and “Med_YawRt” may be        tuned to establish desired performance of the target rejection        filter. Tuning may be accomplished by computer        simulation/modeling and/or by real-world testing.

If either of equation 10 or 11 are true (block 120, “YES”), the allwarnings issued by the FCW system based on detection of roadside objectsare blocked (see block 130). This is, in effect, the equivalent ofreducing the width of the CTZ to zero. The complete block on warningsremains in place at least until the block warning timer has elapsed(block 140, “YES”), at which time the routine loops back through blocks100-120. If the result of block 120 is still “YES,” the warning blockremains in place. The duration for the block warning timer is tunable.

Once neither of eqns. 10 or 11 are true (block 120, “NO”), the methodprogressed to block 150 where the following checks are made:

$\begin{matrix}{\begin{bmatrix}{{{SIA\_ CR}\mspace{14mu}{``{Near}"}} \geq {Min\_ CR}} \\{AND} \\{{{YawRate}\mspace{14mu}{``{Far}"}} \geq {Med\_ YawRt}}\end{bmatrix},} & (12)\end{matrix}$

where “Min_CR”, and “Med_YawRt” are tunable parameters.

The values for all “tunable parameters” referred to herein may bedetermined by testing of real vehicles and/or by computer modeling, in amanner generally as discussed in Example I through Example V below.

If block 150, “YES”, a first width reduction is applied to the CTZ atblock 160. The first width reduction narrows the CTZ sufficiently toachieve an acceptably low level of false warnings caused by roadsideobjects. Similar to the complete warning block of block 130, the firstwidth reduction is maintained for the duration of a first widthreduction timer (block 170). However, if during that time period SIA_CR“Near” increases to the point that SIA_CR “Near”≧Peak_CR (block 180,“YES”), the routine returns to block 130 and all warnings are blocked.

Once the relationships in eqn. 12 are no longer true (block 150, “NO”),the method advances to block 190 and the following conditions arechecked:

$\begin{matrix}{\begin{bmatrix}{{{SIA\_ CR}\mspace{14mu}{``{Near}"}} \geq {Min\_ CR}} \\{AND} \\{{{YawRate}\mspace{14mu}{``{Far}"}} \geq {Min\_ YawRt}}\end{bmatrix},} & (13)\end{matrix}$

-   -   OR

$\begin{matrix}{{\begin{bmatrix}{{{SIA\_ CR}\mspace{14mu}{``{Near}"}} > {Zero\_ CR}} \\{AND} \\{{{SIA\_ CR}\mspace{14mu}{``{Near}"}} < {Min\_ CR}}\end{bmatrix}{{AND}\mspace{14mu}\left\lbrack {{{YawRate}\mspace{14mu}{``{Far}"}} \geq {Med\_ YawRt}} \right\rbrack}},} & (14)\end{matrix}$

where “Min_CR”, “Zero_CR”, and “Min_YawRt” are tunable parameters. Itshould be noted that Zero_CR may not necessarily truly be equal to zero,but rather may be a low value of SIA change rate that may effectively beconsidered the same as zero.

If block 190, “YES”, a second width reduction is applied to the CTZ atblock 200. The second width reduction narrows the CTZ less restrictivelythan the first width reduction (the CTZ with the second width reductionapplied is wider than with the first width reduction applied). The CTZwith the second width reduction is still narrowed enough to reduce theamount/rate of false warnings caused by roadside objects to anacceptably low level for the given conditions.

The second width reduction is maintained until one of the followingconditions is true:

The second width reduction timer has elapsed (block 210, “YES”), and theblock 190 checks return a “NO,” at which point the routine continues toblock 240 (the duration of the second width reduction timer is tunable);or

SIA_CR “Near” increases such that SIA_CR “Near”≧Peak_CR (Block 220,“YES”), at which point the routine returns to block 130 and all warningsare blocked; or

Yaw Rate Change “Far” and SIA_CR “Near” both increase such that Eqn. 12is true (block 230, “YES”), at which point the method returns to block160 and the first width reduction is applied to the CTZ.

When the routine reaches block 240, any one of the warning reductionmeasures may be in force: a) the complete warning block applied at block130, b) the first CTZ width reduction applied at block 160, or c) thesecond CTZ width reduction applied at block 200. That current warningreduction measure is maintained as long as:

$\begin{matrix}{{\begin{bmatrix}{{{SIA\_ CR}\mspace{14mu}{``{Near}"}} > {Zero\_ CR}} \\{AND} \\{{{SIA\_ CR}\mspace{14mu}{``{Near}"}} < {Min\_ CR}}\end{bmatrix}{{AND}\mspace{14mu}\left\lbrack {{{YawRate}\mspace{14mu}{``{Far}"}} < {Med\_ YawRt}} \right\rbrack}},} & (15)\end{matrix}$

-   -   where “Zero_CR”, “Min_CR”, and “Med_YawRt” are tunable        parameters.

The CTZ width is only maintained so long as block 240 returns a YESresult. When block 240 returns a NO result, the method proceeds to block260, where the warning reduction measures are removed and the CTZreturns to its original, unreduced width (block 270). This is true when:[SIA_CR“Near”≦Zero_CR]  (16)

FIG. 5 depicts the Warning Check subroutine that is performedcontinually during operation of the FCW system. Threat warnings areblocked if: The “Block All Warnings” condition from block 130 is inforce; or D_(OFFSET) (the lateral distance of the detected target fromthe predicted host vehicle path) is greater than the CTZ width in forceat the current time. If the D_(OFFSET) of a detected target is withinthe width of the CTZ, then the target is considered valid and a warningcan be provided.

The method disclosed herein is designed to minimize false warnings dueto detection of side-of-path objects during curve entry and decreasingradius curves, while still maintaining positive test performance forsteady state curves where the vehicle yaw rate and subsequent pathprediction are stable. As with any method of this type, trade-offs mustbe made between how aggressively it removes side-of-path objects and howwell it provides positive function on steady curves. To help manage thistradeoff, the method has a number of tunable parameters that affect, invarying degrees, how the filter performs. Five examples of possibletuning strategies are provided here to aid in understanding differenttuning approaches that may be taken.

EXAMPLE I Maximum True Warning in Curves, Minimum False WarningRejection

One possible tuning approach to maximize the ability to provide warningsin curves for true threats is:

Set the SIA-based D_(OFFSET) calculation to predict vehicle yaw rate,when taking the variation of the SIA-based D_(OFFSET) into account,which is less than or equal to the real vehicle yaw rate for themagnitude of steering input.

Set the SIA_CR “Max” threshold to be relatively high, such that warningswill only be blocked for severe steering events.

Set the SIA_CR “Min” and Yaw Rate Change “Far” thresholds to requiregreater steering input and prior yaw rate for achieving the first andsecond filter adjustments.

Set the first and second filter adjustments to make smaller CTZ widthadjustments compared to other tuning options. Note, the CTZ widthadjustment is a ratio of the full CTZ width. Therefore, a smaller CTZwidth adjustment is accomplished using parameter settings that arecloser to 1.0.

May be able to set SIA_CR “Min” to a low value as non-zero SIA whendriving straight will have less effect on the SIA-based D_(OFFSET) dueto the tuning above.

The potential benefits achieved with this type of tuning is removal ofsome false warnings at curve entry where the SIA-based D_(OFFSET) willstill predict that the object will be to the side of path, even thetuning for the D_(OFFSET) calculation is in general less than what theyaw rate based D_(OFFSET) would be. In this case objects close to theroadway in curves would tend to still provide false warnings, but thosefarther away may now be rejected.

EXAMPLE II Some True Warning in Curves; Improved False Warning RejectionThrough Significant CTZ Width Reduction

Another tuning approach to improve false warning rejection, but stillmaintain some true warning capability is to use settings similar toExample 1, except to lower the thresholds required for CTZ widthreduction and to make more significant CTZ width reductions. Compared toExample 1, these changes are

Set the SIA_CR “Min” and Yaw Rate Change “Far” thresholds to requiresteering input and vehicle yaw rate change levels slightly higher thanthat expected during normal straight-line driving and testing for truewarning performance.

Set the first and second filter adjustments to significantly reduce theCTZ width.

If possible, set the Min_SIA_Threshold to a low value (ideally zero) asnon-zero SIA when driving straight will have less effect on theSIA-based D_(OFFSET). A low setting can only be used if the SIA-basedD_(OFFSET) will still be considered an in-path object with theworst-case expected SIA during steady-state straight line driving.

This tuning has the potential to significantly reduce false warningevents at curve entry through narrowing of the CTZ. Some true curverelated warning performance may still be provided for targets that arecentered in the host vehicle's path.

EXAMPLE III Some True Warning in Curves; Improved False WarningRejection Through “Higher Gain” SIA-Based D_(OFFSET)

A third tuning approach is similar to the second example in that it mayprovide some true warning performance in curves. The third approach isdifferent in that it predicts greater SIA-based D_(OFFSET), less CTZwidth reduction, and requires that the Min SIA Threshold be set higherthan the SIA required for straight-line driving. The parameters are setas:

Set the SIA-based D_(OFFSET) calculation to predict vehicle yaw rate,when taking the variation of the SIA-based D_(OFFSET) into account,which varies between equal to and greater than the real vehicle yaw ratefor the magnitude of steering input.

Set the SIA_CR “Min” and Yaw Rate Change “Far” thresholds to lowerlevels than used for Example 1. The Yaw Rate Change “Far” secondthreshold can be just above the yaw rate experienced during normalstraight line road driving. The Yaw Rate Change “Far” first thresholdcan be set slightly higher, but not so high that it is rarely achievedduring curve entry events when “real-world” tests are conducted.

Set the first and second filter adjustments to smaller CTZ widthadjustments compared to Example 2.

The Min_SIA_Threshold must be set above the maximum steady state SIAthat can be expected during straight line driving.

This tuning can provide some true curve related warning performance, butthe performance will degrade as the SIA increases or with suddensteering changes. Also, this approach cannot provide false curve entrywarning rejection for situations where the SIA has not exceeded theMin_SIA_Threshold.

EXAMPLE IV Minimum Warning in Curves; Maximum False Warning RejectionThrough Very High SIA-Based D_(OFFSET)

The fourth example is an extension from Example 3, by setting theSIA-based D_(OFFSET) to be very high. Compared to Example 3, thesettings are:

Set the SIA-based D_(OFFsET) calculation to be very high.

This tuning provides little to no performance in curves as targets willbe immediately calculated as outside the CTZ once the SIA exceeds theMin_SIA_Threshold.

EXAMPLE V Minimum Warning in Curves; Maximum False Warning RejectionUsing Lower SIA_CR Thresholds

This example uses lower SIA_CR thresholds for blocking warnings. Somewarning performance could be provided for curves if the SIA_CR remainslow (e.g., steady steering in the curve). Also, the SIA-based D_(OFFSET)is tuned to ensure all targets are still considered.

Set the SIA_CR “Max” threshold to be slightly above SIA_CR values thatcan be expected for straight line driving and testing.

Set the SIA-based D_(OFFSET) calculation to predict vehicle yaw ratewhich is very low. This essentially removes the SIA-based D_(OFFSET)from rejecting any targets.

This tuning essentially makes the curve entry filter a SIA_CR filter. Itis similar to Example 2 in that lower threshold levels of SIA_CR areused. In Example 2, SIA_CR levels above the thresholds lead to reducedCTZ widths whereas in this example they lead directly to a completeblock of warnings. Under some test conditions, noise on the SIA_CRsignal may prevent setting the SIA_CR threshold low enough to provideexcellent false warning rejection while robustly providing warnings fortrue threat situations when driving straight.

In addition to these tuning examples, there is tuning flexibilityassociated with the signal tracking look-back periods selected for the“far” and “near” signals. For example, the use of longer tracking timesfor SIA_CR “Near” and Yaw Rate Change “Far” leads to larger signalchange outputs as the maximum and minimum are tracked over a longer timeperiod. This affects the key tracked signals as follows:

If a longer tracking/history time is used for SIA_CR “Near”:

Thresholds will be met more quickly and false warnings, therefore, maybe rejected more quickly.

The CTZ width may not reset as quickly due to the signal not droppingbelow the SIA_CR Zero Threshold level as quickly or often.

The time between updates will increase:

$\left\lbrack {{{SWA\_ CR} - {{Time\_ Between}{\_ Updates}}} = \frac{{SWA\_ CR}{\_ Near}{\_ TrackingTime}}{5}} \right\rbrack$

If a longer tracking/look-back period time is used for YawRate “Far”,thresholds will be met more quickly and false warnings may therefore berejected more quickly

The time between updates will increase:

$\left\lbrack {{{YawRate} - {{Time\_ Between}{\_ Updates}}} = \frac{YawRateFar\_ TrackingTime}{10}} \right\rbrack$

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention that may be embodied in variousand alternative forms. The figures are not necessarily to scale; somefeatures may be exaggerated or minimized to show details of particularcomponents. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as arepresentative basis for teaching one skilled in the art to variouslyemploy the present invention.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms of the invention. Rather,the words used in the specification are words of description rather thanlimitation, and it is understood that various changes may be madewithout departing from the spirit and scope of the invention.Additionally, the features of various implementing embodiments may becombined to form further embodiments of the invention.

What is claimed is:
 1. A method of forward collision warning for a motorvehicle comprising: operating a sensor system to detect a target aheadof the vehicle; identifying a Collision Threat Zone (CTZ) along apredicted path of the vehicle; continuously measuring a steering inputangle (SIA) change rate and a yaw rate of the vehicle; continuouslytracking a near-past maximum absolute value of the SIA change rateduring a first look-back period; continuously tracking a far-pastvariation of the yaw rate during a second look-back period longer thanthe first look-back period; applying a first width-reduction to at leasta portion of the CTZ in the event that a) the near-past maximum absolutevalue of SIA change rate exceeds a first SIA change rate threshold, andb) the far-past variation of yaw rate exceeds a first yaw rate thresholdvalue; and suppressing a threat warning based on detection of the targetif the target is outside of the CTZ with the first width-reductionapplied.
 2. The method of claim 1 further comprising: continuouslymeasuring a driver brake application; continuously tracking a near-pastchange in the driver brake application during a third look-back periodshorter than the second look-back period; suppressing the threat warningif: a) the near-past change of the driver brake application exceeds afirst DBA threshold value; and b) at least one of the near-past maximumabsolute value of SIA change rate and the far-past variation of yaw rateexceed the first SIA change rate threshold and the first yaw ratethreshold value respectively.
 3. The method of claim 1 furthercomprising: suppressing the threat warning if the near-past maximumabsolute value of SIA change rate exceeds a peak value higher than thefirst SIA change rate threshold.
 4. The method of claim 1 furthercomprising: applying a second width-reduction to at least a portion ofthe Collision Threat Zone, the second width-reduction being lessrestrictive than the first width reduction if: a) the near-past maximumabsolute value of SIA change rate exceeds the first SIA change ratethreshold; and b) the far-past variation of yaw rate exceeds a secondyaw rate threshold lower than the first yaw rate threshold; andsuppressing the threat warning if the target is outside of the CTZ withthe second width-reduction applied.
 5. The method of claim 1 furthercomprising: applying a second width-reduction to at least a portion ofthe CTZ, the second width-reduction being less restrictive than thefirst width reduction, if: a) the near-past maximum absolute value ofSIA change rate is less than the first SIA change rate threshold butexceeds a second SIA change rate threshold lower than the first SIAchange rate threshold; and b) the far-past variation of yaw rate exceedsthe first yaw rate threshold; and suppressing the threat warning if thetarget is outside of the CTZ with the second width-reduction applied. 6.The method of claim 1 wherein the predicted path of the vehicle iscalculated by estimating a future yaw rate based on measured values of avehicle speed and a steering input angle.
 7. The method of claim 1wherein the first width reduction is removed if the conditions forapplying the first width reduction do not exist at the end of a timeperiod.
 8. The method of claim 1 wherein the first width reductioncomprises reducing the CTZ width to zero.
 9. A method of forwardcollision warning for a motor vehicle comprising: operating a sensorsystem to detect a target ahead of the vehicle; identifying a CollisionThreat Zone along a predicted path of the vehicle; continuouslymeasuring a steering input angle (SIA) change rate, a yaw rate, and adriver brake application; continuously tracking a near-past maximumabsolute value of the SIA change rate during a first look-back period;continuously tracking a far-past variation of the yaw rate during asecond look-back period longer than the first look-back period;continuously tracking a near-past variation of the driver brakeapplication during a third look-back period shorter than the secondlook-back period; and suppressing threat warnings related to the targetif: a) the near-past variation of the driver brake application exceeds aDBA threshold; and b) the near-past maximum absolute value of SIA changerate exceeds a first SIA change rate threshold OR the far-past variationof yaw rate exceeds a medium yaw rate threshold.
 10. The method of claim9 further comprising: suppressing threat warnings related to the targetif the near-past maximum absolute value of SIA change rate exceeds apeak value higher than the first SIA change rate threshold.
 11. Themethod of claim 9 further comprising: applying a first width reductionto the CTZ if: a) the near-past variation of the driver brakeapplication does not exceed the DBA threshold; and b) the near-pastmaximum absolute value of SIA change rate exceeds the first SIA changerate threshold; and c) the far-past variation of yaw rate exceeds themedium yaw rate threshold value; and suppressing a threat warning if thetarget is outside of the CTZ with the first width reduction applied. 12.The method of claim 9 wherein the predicted path of the vehicle iscalculated by estimating a future yaw rate based on measured values of avehicle speed and a steering input angle.
 13. The method of claim 9wherein the first width reduction is removed from the CTZ if theconditions for applying the first width reduction do not exist at theend of a time period.
 14. The method of claim 9 wherein the first widthreduction comprises reducing the CTZ width to zero.
 15. A methodcomprising: operating a forward collision warning sensor of a motorvehicle to detect a target ahead of the vehicle; identifying a collisionthreat zone (CTZ) ahead of the vehicle in which the target is considereda collision threat, the CTZ having a base width; continuously measuringa steering input angle (SIA) change rate, a driver brake application(DBA), and a yaw rate; finding a degree of near-past DBA during a firstlook-back period; finding a degree of near-past SIA change rate during asecond look-back period; finding a degree of far-past yaw rate during athird look-back period longer than the first and the second look-backperiods; applying a first width reduction to the CTZ if either 1) thenear past SIA change rate exceeds a peak SIA change rate threshold, or2) the near-past DBA indicates braking and either a) the near past SIAchange rate exceeds a second SIA change rate threshold lower than thepeak SIA change rate threshold or b) the far-past yaw rate exceeds afirst yaw rate threshold; and suppressing a threat warning if the targetis outside of the CTZ with the first width reduction applied.
 16. Themethod of claim 15 further comprising: in the event that the conditionsto apply the first width reduction are not met, applying a second widthreduction less restrictive than the first width reduction to the CTZwhen: a) the near-past maximum absolute value of SIA change rate exceedsthe second SIA change rate threshold, and b) the far-past variation ofyaw rate exceeds the first yaw rate threshold value; and suppressing thethreat warning if the target is outside of the CTZ with the secondwidth-reduction applied.
 17. The method of claim 16 further comprising:in the event that the conditions to apply the first width reduction arenot met and the conditions to apply the second width reduction are notmet, applying a third width reduction less restrictive than the secondwidth reduction to the CTZ when: a) the near-past maximum absolute valueof SIA change rate exceeds the second SIA change rate threshold; and b)the far-past variation of yaw rate exceeds a second yaw rate thresholdlower than the first yaw rate threshold; and suppressing the threatwarning if the target is outside of the CTZ with the thirdwidth-reduction applied.
 18. The method of claim 16 further comprising:in the event that the conditions to apply the first width reduction arenot met and the conditions to apply the second width reduction are notmet, applying a third width reduction less restrictive than the secondwidth reduction to the CTZ when at least one of: a) the near-pastmaximum absolute value of SIA change rate is less than the second SIAchange rate threshold but exceeds a third SIA change rate thresholdlower than the second SIA change rate threshold; and b) the far-pastvariation of yaw rate exceeds the first yaw rate threshold; andsuppressing the threat warning if the target is outside of the CTZ withthe third width-reduction applied.
 19. The method of claim 15 whereinthe first and second look-back periods are the same duration.
 20. Themethod of claim 15 wherein the first width reduction results in the CTZhaving a width of zero.